In telecommunications systems, the vehicle that will most likely be used for offering a wide range of different high-bandwidth services, e.g., multimedia services, will most likely be based on Asynchronous Transfer Mode (ATM) protocols. These protocols define a particular data structure called a "cell", which is a data packet of a fixed size (53 octets, each octet comprising eight bits). A cell is formed by a header (five octets) and payload (48 octets) for transporting routing and user information.
The cell-routing concept in ATM is based on two aspects comprising a routing field in the cell header containing a Virtual Path Identifier (VPI) and Virtual Channel Identifier (VCI). The VCI and VPI pair have only local significance on the link between ATM switches (nodes). ATM switches as well as so-called cross-connect apparatus use routing tables to map VCI and VPI values received via an incoming link to outgoing values and an outgoing link as a way of routing the associated cell through the ATM switch (or cross-connect apparatus). A virtual Circuit Link (VCL) is a logical link between two switches (or a cross-connect nodes) and is identified by a VCI value. Similarly, a Virtual Path Link (VPL) is a logical link between two switches (or cross-connect nodes) identified by a respective VPI value. A virtual Circuit Connection (VCC) is an end-to-end connection between two devices and is formed by the concatenation of VCLs, and a Virtual Path Connection (VPC) is formed by the concatenation of VPLs. If an appreciable number of VCCs follow the same route segment, then it is likely that they will share the same VPC associated with that segment. In such a case, intermediate switches do not change the VCI values, and, therefore, are referred to as VP switches.
Current telephone networks as well as their associated transmission media, routing and cross-connection devices are digital circuit switched facilities, in which the routing of user information, e.g., voice and voice band-data services, from a source to a destination is via an end-to-end switched connection, which is dedicated for the duration of an associated call using the connection. That is, the call is set-up by assigning it to an idle time slot (one for each link) of a frame of time slots that are transported over a digital link(s) interconnecting origination and destination switches. As such, the connection is semi-permanent--lasting only for the duration of the associated call.
In contrast, an ATM network does not use dedicated time slots. Instead, VCCs and VPCs share the network resources asynchronously. An ATM network thus has to ensure that its resources are sufficient to handle the traffic that is transported via the VCCs and/or VPCs that it has established (set up).
It is well-known that current circuit-switched voice and voice-band data services use one of a number of different signaling and messaging techniques for the purpose of establishing a circuit switched connection between Synchronous Transfer Mode (STM) switches or accessing network databases to process special telephone services. Such signaling techniques include in-band signaling using so-called "borrowed bits" associated with a data stream; in-band signaling using Multi-Frequency (MF) tones, and out-of-band signaling using a separate packet network. In-band signaling using "borrowed bits" is used by customer premises equipment (e.g., a private branch exchange) to signal an STM network switch over a digital transmission facility. MF in-band signaling is still used in some of the switches associated with Local Exchange Carriers (LEC) to set up a call connection, but such signaling is being replaced by out-of-band packet signaling, for example, the packet signaling provided by the well-known Signaling System 7 (SS7). SS7 signaling is used by Interexchange Carrier (IXC) networks (e.g., AT&T) to establish call connections over their associated intertoll digital networks and to access network databases. Advantageously, most, but not all, LEC switches are now being provided with the SS7 type of out-of-band signaling capability.
Network switches perform other functions in addition to signaling. These other functions include, for example, Digital Signal Processing (DSP) functions such as detecting special tones, playing recorded announcements, canceling echoes, etc.
Presently, the designers of telecommunications networks are seriously considering replacing the STM switching and associated transport facilities with Broadband ISDN (B-ISDN) based on ATM as the underlying technology. What this means is that the circuit switched structure, associated signaling systems, databases, operations systems, etc., will be replaced by systems using ATM based transport, signaling and messaging. At this point in time, it appears that changing the STM switched transport to ATM transport may be relatively easy and could be accomplished in the near future. However, network signaling and messaging have been designed and developed over many years to guarantee that critical network applications will operate correctly. It is therefore unlikely that the entire signaling network will be converted at once to broadband signaling. It is also unlikely that a telecommunications carrier (LEC or IXC) will replace its entire STM network at once with a B-ISDN/ATM network, but will more likely migrate toward that end in stages such that during intermediate stages of the conversion network may be composed of STM and ATM elements. Accordingly, there will be a need to develop technology that will gracefully interface STM elements with ATM elements and allow ATM type switches to interface with the different types of existing signaling networks. The need for such interfacing has been recognized, but has been limited to the transport of user information only. Accordingly, the relevant technology has only advanced to the point of defining a Terminal Adapter (TA) function to implement appropriate ATM Adaptation Layer (AAL) protocols to interface conventional circuit-switched-transport protocols (e.g., time slot protocols for voice and dedicated circuits) with ATM based transport protocols.